The Third Generation Partnership Project (3GPP) Technical Specification (TS) 36.423 details the “X2” interface, used between radio base stations in Evolved Terrestrial Radio Access Networks (EUTRAN). A base station in the EUTRAN context is termed an “eNB” or “eNodeB.” Among other uses, the X2 interface provides for the distribution of own-cell configuration data to neighboring eNBs.
For each cell, the cell's own “served” Public Land Mobile Networks or PLMNs are signaled to the neighboring cells. In other words, a given radio cell may provide access to more than one PLMN, and each such PLMN may be associated with a different network operator. Often, within the multi-operator scenario, a given RAN cell supports two or more core networks, where each network is associated with a different operator.
However, even where a given cell is associated with multiple core networks, at any given time not all such networks may be available for serving user equipment (UE). As one example, communication with the Mobility Management Entity (MME) or other element in an associated core network may be lost, at least temporarily. Or, some functionality within a given core network may be temporarily impaired. In such cases, the PLMN associated with that network is considered unavailable for serving UEs (e.g., mobile terminals and other communication devices).
When a served PLMN is lost at a given cell, the PLMN is removed from the system information broadcast (SIB) transmitted by the cell, to prevent UEs associated with that PLMN from attempting to camp in the cell. The loss is also indicated to neighboring cells over the X2 interface, such as by removing the IDs of unavailable PLMNs from the cell information update messages that are sent between eNBs. Making the loss visible to neighboring cells prevents, for example, those neighboring cells from attempting (prepared or blind) handover of UEs into the cell, for PLMNs that are unavailable in the cell.
As a general proposition, each cell maintains some form of neighbor list information, such as one or more tables or other data structures, including entries for each neighboring cell, its ID, and the IDs of the served PLMNs that are available in the neighboring cell. The cell also maintains certain performance data, such as handover performance data that indicates in one sense or another handover performance from or to the neighboring cell. Additionally, or alternatively, the same or other performance data may be maintained in an operations and maintenance node, which may store a listing of associated cells and, for each such cell, neighboring cell data. When the last served PLMN available in a given cell is lost, that cell is removed from neighbor cell listings in the surrounding cells, as it no longer offers any served PLMNs for use by UEs. Commonly, when a cell is removed from the neighbor list in a given base station, that base station also removes performance or other data that is specific to the removed cell. Further, such cell data deletions may be propagated to operations and maintenance (O&M) entities.